Recording materials have been disclosed on which records are made thermally by the use of light beams like laser having a high energy density. In such thermal recording materials, information is recorded by creating differences in optical density on the recording layer. The recording layer has high optical density and absorbs light beams of high energy density which impinge thereon. The absorption of light brings about a local temperature rise, causing a thermal change such as melting-cohesion or evaporation to take place in the recording layer. As a result, the irradiated parts of the recording layer are removed, and a difference in optical density is formed between the irradiated parts and the unirradiated parts (cf. U.S. Pat. Nos. 4,216,501, 4,233,626, 4,188,214 and 4,291,119 and British Pat. No. 2,026,346).
These thermal recording materials usually do not require development and fixing processes and do not require darkroom operations because of their insensitivity to room light. In addition, they provide high-contrast images and make it possible to add information later on.
Recording on such thermal recording materials is usually accomplished by converting the information to be recorded into electrical time series signals and scanning the recording material with a laser beam which is modulated in accordance with the signals. This method is advantageous in that recording images can be obtained on real time (i.e. instantaneously).
The recording layer of such thermal recording materials is usually made of inexpensive metals, dyes, or plastics. Recording materials like this are described in "Electron, Ion, and Laser Beam Technology", by M. L. Levene et al.; The Proceedings of the Eleventh Symposium (1969); "Electronics" (Mar. 18, 1968), P. 50; "The Bell System Technical Journal", by D. Maydan, Vol. 50 (1971), P. 1761; and "Science", by C. O. Carlson, Vol. 154 (1966), P. 1550.
In accordance with previously used methods, some of the metal recording layers are formed by depositing a thin film of Bi, Sn, or In on the substrate. Such recording layers are superior as thermal recording materials because they permit the recording of images with good resolution at high contrast. However, they are disadvantageous in that the metal thin film reflects more than 50% of the laser light, wasting the energy of the laser light. Accordingly, such material may require a substantial amount of energy for recording. Therefore, a high output laser light source is required if records are to be made by high-speed scanning. This makes it necessary to use a large, expensive recording apparatus.
In order to overcome the above drawbacks, efforts have been made to find recording materials having high sensitivity. An example of such a recording material is disclosed in Japanese Patent Publication No. 40479/1971. According to this disclosure, the recording material is made up of three thin films of Se, Bi, and Ge. The extremely thin top layer of Ge is provided to reduce reflection by the thin films of Se and Bi. Nevertheless, they are not capable of producing recorded images of satisfactory quality.
There is disclosed in Japanese Application (OPI) No. 74632/1976 (the term "OPI" as used herein refers to a "Published unexamined Japanese Patent Application) another example of recording materials having an antireflection layer. According to this disclosure, the metal layer is coated with an anti-reflection layer which absorbs the laser light of specific wavelengths for recording. However, such an arrangement is not desirable because the antireflection layer reduces the reflectivity of the unrecorded parts. The small difference in reflectivity between the recorded parts and unrecorded parts makes it difficult to read the recordings by means of reflected light.
A recording layer comprised of a highly reflective metal is suitable for reading recordings by means of reflected light. However, such a recording layer posesses low sensitivity with respect to information recording. On the other hand, a recording layer which is rendered highly sensitive by providing an antireflection layer is not suitable for reading recordings by means of reflected light. A proposal for dealing with these conflicting characteristics was proposed in Japanese Patent Application (OPI) Nos. 120506/1978 (corresponds to U.S. Pat. No. 4,216,501) and 122403/1978 (corresponds to U.S. Pat. No. 4,233,626) which disclose a recording material which includes a dielectric layer and a bottom reflection layer placed between the recording layer and the substrate. According to these disclosures, the recording layer is made of a metal such as rhodium which has a high reflectivity. Therefore, if the recording sensitivity is to be increased and the records are to be read by reflection at high contrast, the recording material should be of the nega-posi type which produces positive images from negative originals. Such nega-posi type can be produced by strictly defining the thickness of each layer according to the wavelength of the light used for reading so that the reflectivity of the light used for reading is minimized.
As mentioned above, conventional recording materials are not desirable because accurate control is required in order to make the film layers. Furthermore, the light for reading the records is limited with respect to wavelength, and the incident angle of light for reading the records is limited.